1. Field of the Invention
The present invention relates to a planar microlens array used in a liquid crystal display element and so on.
2. Description of the Related Art
Prior-art liquid crystal display elements in which a planar microlens array and a liquid crystal layer are combined are shown in FIG. 1(A) and FIG. 1(B) of the accompanying drawings. The liquid crystal display element shown in FIG. 1(A) includes a planar microlens array 1 comprising an array of convex microlenses 3 provided on a surface of a base glass plate 2. The array of convex microlenses 3 is covered with a cover glass plate 4 which is bonded to the array of convex microlenses 3 by an adhesive layer 5. A liquid crystal layer 7 is filled between the cover glass plate 4 and a TFT (Thin Film Transistor) glass substrate 6. The TFT glass substrate 6 supports transparent pixel electrodes 8 on its surface facing the liquid crystal layer 7. The surface of the TFT glass substrate 6 includes areas 9 that are free of the transparent pixel electrodes 8 and carry interconnections and TFTs which do not pass applied light. Electrodes 10 which confront the transparent pixel electrodes 8 are mounted on a surface of the cover glass plate 4 that faces the liquid crystal layer 7.
In the liquid crystal display element shown in FIG. 1(B), an array of convex microlenses 3 is provided on a surface of a cover glass plate 4.
A planar microlens array according to the present invention can be applied to both the liquid crystal display elements shown in FIG. 1(A) and FIG. 1(B).
The planar microlens array 1 operates as follows: Applied light is converged by the convex microlenses 3 onto the transparent pixel electrodes 8 to brighten an image projected onto a screen.
A process of manufacturing the planar microlens array 1, the structure of which is mentioned above, is as follows: As shown in FIG. 2(A), a release agent is coated on a shaping surface of a stamper 11 on which convex portions are densely arranged, and a light-curable or heat-curable synthetic resin material having a high refractive index is set on the shaping surface of the stamper 11. Next, as shown in FIG. 2(B), the base glass plate 2 is pushed onto the synthetic resin material, thereby spreading the synthetic resin material, and the synthetic resin material is cured by applying ultraviolet radiation or heating and is shaped to form the convex microlenses 3. Thereafter the stamper 11 is peeled off.
Next, as shown in FIG. 2(C), a light-curable or heat-curable synthetic resin material having a low refractive index is coated onto the convex microlenses 3, and a glass substrate which is made into a cover glass plate 4 is pushed onto the synthetic resin material, thereby spreading the same. Thereafter, the synthetic resin material is cured and finally the planar microlens array 1 is formed by grinding the glass substrate to the thickness of the cover glass plate 4.
The convex microlenses may be formed on the glass substrate.
Presently available liquid crystal display panels have pixel dimensions ranging from about 40 .mu.m to 60 .mu.m. It is expected that the pixel dimensions will be reduced to about 20 .mu.m to 30 .mu.m in the future to meet demands for clearer displayed images.
Smaller pixel dimensions require the convex microlenses 3 to be reduced in size, resulting in a shorter focal length. For efficient utilization of the applied light, it is necessary that the focal point of the convex microlenses 3 be positioned substantially on the transparent pixel electrodes. To meet such a requirement, the cover glass plate 4 must be reduced in thickness.
Each of the convex microlenses 3 and the adhesive layer 5 is made of a heat-curable or ultraviolet-curable synthetic resin. The synthetic resin shrinks when cured. In particular, the synthetic resin having a high refractive index which makes the convex microlenses 3 has high Young's modulus and high residual stress.
The cover glass plate 4 can withstand the shrinkage of the synthetic resin, provided that the cover glass plate 4 has a substantial thickness. However, if the cover glass plate 4 is thinner, it tends to yield and allow the entire planar microlens array 1 to warp upon shrinkage of the synthetic resin, as shown in FIG. 3 of the accompanying drawings. In particular, shrinkage of the synthetic resin having a high refractive index may result in high residual stress. As a result of this, the width of a cell gap which is formed between the planar microlens array 1 and the TFT glass substrate 6 and to which liquid crystal is applied varies between the center and the periphery. Presently the maximum permissible error range of a cell gap in size is 1.5 .mu.m.
On the other hand, the Young's modulus of the synthetic resin having a low refractive index which makes the adhesive layer 5 is smaller than that of the synthetic resin having a high refractive index which makes the convex microlenses 3. That does not mainly cause the warpage but does mainly cause the small voids.
Namely, as shown in FIG. 4, small voids are produced between the convex microlenses 3 and the adhesive layer 5 because the volume shrinkage percentage of the synthetic resin having a low refractive index generally reaches to 6-9%, and further the synthetic resin having a low refractive index has small values in membrane intensity and interface adhesion intensity.